Predictive-model-based MIMO motion control of an unstable 6-DoF hexapod stage for overlay measurements

Correct functionality of semiconductor devices depends on the overlay performance between device layers. Future smaller device features consequently require more accurate overlay metrology tools. In this paper we present a large dynamic range AFM overlay tool to directly measure marker-to-feature distances, and focus on the controller design of the large stroke 6-DoF hexapod motion stage which is at the heart of this tool. The stage is open-loop unstable due to magnetic gravity compensation, which calls for an initial model-based MIMO controller design, which is based on a finite-element modal analysis, geometric decoupling and SISO loopshaping. This controller is successfully validated in experiments; further controller improvements after closed-loop system identification yield a closed-loop position error of just 1 nm RMS.